A store of liquefied natural gas, and in particular an underground store, suffers from an evaporation rate of about 0.5% per day, regardless of the nature of the rock formation in which the store is hollowed out. This value corresponds to storage conditions at temperatures lying in the range -162.degree. C. to about -100.degree. C.
Given that natural gas is not a pure gas but a mixture of different gases, and that the store contains two phases, the composition of the gas phase above the liquid phase will be different from the composition of the liquid phase itself.
However, in order to maintain the storage temperature and pressure conditions substantially constant, it is necessary to provide some means for allowing a certain rate of evaporation, and thus for substantially permanently allowing a portion of the stored gas phase to escape in order to compensate for the influx of heat, which in the case of an underground store comes from the the surrounding rock formation.
In an above-ground store associated with a unit for smoothing demand peaks, the waiting period is very long, and as a result the above-described phenomenon of the liquid phase becoming enriched in heavy fractions takes place in a manner similar to an underground store, even though the daily evaporation rate is lower.
Thus releasing a portion of the gas phase has the effect, in the long run, of modifying the composition of the stored product and in some cases this may lead to severe problems which need solving. For example, the composition of the stored product determines its heating power which in turn, for natural gas, determines the dimensions of the burners which can be fed with said gas, or the fuel consumption of said burners.
Preferred implementations of the present invention provide a method for maintaining a liquefied gas in storage at a constant composition.